GM S-Series 4WD Troubleshooting
With either option, two things are happening when you push the 4WD
button or throw the lever into 4WD. First, the transfer case shifts and
delivers power to the front drive shaft. With push-button 4WD, the
shifting is done by a shift motor that is mounted to the transfer case.
With manual 4WD, the lever on the floor (above right) is what shifts
the transfer case.

This alone will not give you 4WD. The second action that has to take
place is the front wheel axles must be joined together inside the
differential housing. This gets the front wheels turning by way of the
front drive shaft. The best I understand it, a collar must slide across
the two axle shafts and join them up. When the collar slides and the
axles are joined, the front drive shaft will power the front wheels (a
very lay person's explanation...).

So, transfer case engagement plus front hub lock equals 4WD. Let's
take a look at troubleshooting the mechanical and electronic
components of the 4WD system.

Mechanical Troubleshooting
The most common cause of non-functioning 4WD? Vacuum leaks.
What, you say, does vacuum have to do with 4WD? Well, the auto
designers of the world always knew that internal combustion engines
generate a good deal of vacuum, which can be used for a variety of
purposes. It's free energy, more or less, and can be transferred easily
throughout the vehicle. Over the years, General Motors and most
other car and truck makers have put vacuum to use in powering
certain vehicle systems. For example, in the HVAC system of S-series
trucks, most of the well-hidden doors and chambers which direct air
to the proper vents are powered by vacuum.

Vacuum is also used to pull the cable that slides the collar inside the
front differential housing and joins the axle shafts. Without vacuum,
the cable won't move. The transfer case can do its thing, but if the
cable doesn't pull the collar, 4WD is just a pipe dream. That cable is
pulled by a pneumatic actuator under the battery tray. The actuator
gets its vacuum when the transfer case shifts to 4WD. Shift forks, or
whatever they call this apparatus inside the transfer case, push up
against a spring-loaded ball that is part of the transfer case vacuum
switch.
At left is the push-button 4WD from my 1996 GMC Sonoma. This was an
option; standard 4WD trucks came with a manual shifter on the floor, as
pictured on the right.
First off, a disclaimer: I am not a mechanic. I have never been a
mechanic. Anything you read here or anywhere else on this website is
based on my own personal experiences, or the massive amounts of
information I've gleaned from the Internet on whatever topic I choose
to ramble about. If you've found this website, you were probably in
need of help, or you were really, really bored. What follows is yet
another of my attempts to share what I've learned, or at least point
you in a better direction. Or, simply entertain you with my
unsophisticated descriptions of what goes on inside GM S-series
vehicles. In this case, the topic is four wheel drive.

Every year around Thanksgiving, the S-10/Blazer/Sonoma/Jimmy
forums are abuzz with questions about non-functioning four wheel
drive. Snow comes, and at the worst possible time, no 4x4...very
annoying. It's happened to me. As I've researched the 4WD system
on these vehicles, the problems really come down to one of two
things: 1) Something mechanical; or 2) Something electronic. Now, if
you don't have push-button 4WD on your dash, the electronic part of
this is a lot simpler. If you do have those convenient buttons, you'll
have a couple other components to troubleshoot if your 4WD gives
out.

All three of the S-series vehicles I've owned had electronic shift
transfer cases. These are the trucks with those handy dash-mounted
buttons that, with one push, give you 4WD on the fly. If your truck
doesn't have those buttons, you've got a lever on the floor.
This switch, pictured above left, sits on top of the transfer case. The
ball on the bottom is what is pushed up by the internals of the
transfer case. There are 3 tubes sticking out the top, each connected
to a vacuum line. One line comes from the vacuum source under the
hood; another line goes to the actuator under the battery tray
(pictured above right); and the third line is a vent line that tees off into
2 separate lines. When the ball is pushed, it opens a valve that lets
the vacuum through the switch and to the actuator.

Sounds fairly simple, right? Well, vacuum can be finicky as trucks
age. The rubbery vacuum lines eventually break down and begin to
leak - especially the ones under the hood that are subject to engine
heat. Usually there's a high correlation between non-functioning 4WD
and non-functioning HVAC controls inside the cab, because both
systems use vacuum.

Besides leaks in the vacuum lines, another common reason for loss of
vacuum is sometimes the line becomes disconnected. One of the
most common of these is the vacuum line that connects to the
vacuum reservoir. On my 1996 Sonoma, whose reservoir was a plastic
ball attached to the underside of the hood, a disconnected line is
pretty easy to spot. On my 2004 Blazer, the reservoir is tucked inside
the driver's side fender. The connection is just about impossible to
see, so sometimes people will notice a disconnected vacuum line,
don't know were it goes, and simply cap it off. That might get the
vacuum system functional again, but in certain driving conditions
there won't be enough reserve supply of vacuum to control the HVAC
system or keep the front hub locked.
Besides non-leaking vacuum lines, obviously the actuator itself has to
be working properly if 4WD is going to function. The rubber
diaphragm can't have any holes or rips. It also shouldn't be full of
transfer case fluid. Why would it be full of transfer case fluid? Well,
read all about it. If that's the case, then the transfer case vacuum
switch is bad, either by letting transfer case fluid into the vacuum
lines or by sticking open. It the vacuum switch is stuck open, the
actuator will continuously pull on the hub-lock cable. That's not really
good for the front end and can eventually result in a grinding noise.
Long-term, here's what can happen:
Grinding Gears. Or, this can
happen.

And speaking of actuators, there are several inside the dash, each
controlling a door or flap that directs air to the proper vents. If any of
these are leaking, there may not be enough vacuum reserve to
overcome the leak and keep the 4WD actuator pulling on that cable.
For this reason, troubleshooting your 4WD vacuum issues may not be
as simple as checking for leaks in the vacuum lines or in the actuator
under the batter tray. You may need to look at the vacuum lines and
the actuators in the dash.

Electronic Troubleshooting
For trucks with an electronic shift transfer case, electronic gremlins
can rear their invisible heads and cause disfunction in 4WD shifting.
The electronics are made up of three primary components: 1) Transfer
case control module ("TCCM"); 2) Dash buttons; and 3) Transfer case
shift motor (sometimes called an "encoder").

The brains of the operation is the TCCM, located behind the
passenger side kick panel under the dash. When you push one of the
4WD buttons on the dash, the TCCM tells the shift motor what to do.
It also keeps track of faults in the system, and will eventually shut
down the shift motor if enough faults are measured.

The transfer case shift motor does the grunt work. It shifts the transfer
case into one of three positions: 4-Hi, 4-Lo, and back to 2-Hi. This is
pretty much what the floor lever does, if your truck doesn't have
electronic transfer case shifting.

The shift buttons on the dash are there not only to request that the
TCCM tell the shift motor what to do, but also to help you
troubleshoot the TCCM. This is where the GM service manual comes
in handy. I've transcribed the GM instructions for troubleshooting the
3-button 4x4 system:
The vacuum reservoir "ball" is shown on the left. The vacuum cannister from
the 2004 Blazer is on the right. It fits into a well-hidden space in the fender well.
The vacuum line connection is facing down.
Pneumatic
actuator --->
Cable to front differential -->
Basically, if you push one of the buttons on the dash and they all
blink for a short time and you don't get the action you were expecting,
the TCCM is telling you that it's recorded enough faults that there's
something wrong. It won't let you shift the transfer case. In the
troubleshooting guide, there is a procedure where you can make the
TCCM tell you what's wrong. It communicates the faults by making
the dash buttons blink on and off. Depending on how many blinks
you get, and the sequence of the blinks, the TCCM will identify what
part of the electronics isn't working correctly.

The TCCM identifies one (or more) of four Diagnostic Trouble Codes
("DTCs"), each with a troubleshooting procedure. Some of the more
common problems are a faulty transfer case shift motor, a corroded
electrical connection on the TCCM, and a bad TCCM. One out of
those three is a cheap fix. The other two are a bit more expensive.

The best part of reading DTCs is that it gives you a shot at narrowing
down the problem. I've read a lot of Internet chatter from guys who
just start replacing this or that, without knowing exactly what the
problem is. I've never had any problems from my 4x4 electronics, so I
can't speak to how well the troubleshooting guide works, but it's there
for your reference. Enjoy!